Circuit for operating a bar graph display panel

ABSTRACT

A bar graph display panel comprises a gas-filled envelope containing a series of fine, closely-spaced cathode lines connected in groups so that they can be energized sequentially and individually. The panel also includes an anode film supported on the face plate and aligned with and in operative relation with the series of cathode lines. The anode and cathodes are closely spaced and the panel contains an ionizable gas at relatively high pressure. 
     In order to use the display panel to display a bar of light, the anode and cathodes are connected in an operating circuit wherein each of the cathodes is energized or scanned and caused to glow sequentially. An analog signal to be displayed as a bar of light and a ramp signal are coupled to a comparator, the output of which energizes the anode electrode. When the two input signals are equal, the output of the comparator is zero, the anode is cut off, and the time in the scanning cycle when this occurs determines the number of cathodes which have been energized and the length of the bar of light displayed thereby. The scanning operation is carried out continually at such a rate that the bar of light appears to be stationary but is changeable in accordance with changes in the input analog signal.

BACKGROUND OF THE INVENTION

The principles of the invention are particularly useful for displayinganalog signals, and, although devices are known in the prior art forproviding a visual display of analog signals, none of these devices isas compact and as simple in construction and operation as the device ofthe invention. In addition, prior art devices and circuits for operatingthem do not provide a display of a continuous bar of light whose lengthis changeable.

DESCRIPTION OF THE DRAWINGS

FIG. 1 is a plan view of a display panel embodying the invention;

FIG. 2 is a sectional view along the lines 2--2 in FIG. 1; and

FIG. 3 is a schematic representation of the display panel of FIG. 1 anda drive circuit therefor embodying the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

A display panel 10 embodying the invention is of generally elongatedrectangular shape and includes an insulating base plate 20 of glass,ceramic, or the like, and a face plate 30 of glass or the like andcomprising a viewing window for the panel. The base plate and face plateare hermetically sealed together along a predetermined perimeter bymeans of a glass frit seal (not shown), as is well known in the art.

The base plate 20 includes top and bottom edges 22 and 24, respectively,and left and right side edges 26 and 28, respectively, (FIG. 1). Thebase plate also includes a top surface, on which is formed alight-absorbing, opaque, preferably black, insulating layer 44 which isprovided to optimize the viewing contrast of the panel. A series ofshort, closely spaced, parallel lines 50A, 50B, 50C of a conductivematerial are formed on the layer 44, preferably by a silk-screenprocess. The conductive lines are sufficiently close so that, when thelines are operated as cathodes, the glow of one cathode will appear tomerge with the glow of an adjacent cathode. Typically, the cathode lineshave a width of about 10 to 12 mils, and they are spaced apart about 5to 8 mils.

In one arrangement, the conductive lines 50 are oriented perpendicularto the long axis of the base plate, and they are connected in groups,with the lines 50A, 50B, and 50C being in separate groups. Thus, eachtwo A lines are separated by B and C lines, each two B lines areseparated by A and C lines, and each two C lines are separated by A andB lines.

All of the A lines are connected to a single common conductor 60A whichextends parallel to the longitudinal axis of the base plate to the loweredge 24 where it terminates in a contact pad 70A. Similarly, lines 50Care connected to a longitudinal common conductor 60C which terminates ina terminal pad 70C at edge 24 of the base plate. An insulating layer 80is formed over conductor 60A, with apertures 84 overlaying each of thelines 50B, and a single longitudinal conductor 60B is formed on theinsulating layer 80 in contact with each line 60B and terminating interminal pad 70B. Insulation 80' (FIG. 2) covers line 60B.

A similar insulating layer 83 is formed over the conductor 60C andportions of the conductive lines 50C. In addition, the layers 80 and 83are spaced apart a suitable distance to provide a desired visible lengthof each of the conductive lines 50A, B, and C between them. The exposedlengths of these conductive lines, shown as solid black lines in FIG. 1,are the portions which glow in operation of the panel.

The panel includes an auxiliary "reset" cathode line 50R positionedparallel and adjacent to the first line 50A in the series. Theconductive line 50R is connected to contact pad 70R, and it is turned onfirst before the adjacent line 50A in an operating cycle to provideexcited particles which insure that the first line 50A itself will turnon when it is electrically energized. In addition, the panel includes akeep-alive cell 85 comprising a small-area anode 50KA and a small-areacathode 50KK disposed adjacent to the lower edge 24 of the base plateand connected to suitable terminal pads 60KK and 60KA. The keep-aliveelectrodes are positioned in close proximity to reset cathode 50R toprovide excited particles therefor. Usually, the keep-alive electrodesare connected to a power source V and are always ON and generatingexcited particles.

The face plate of the panel includes, on its lower surface, atransparent conductive film anode 90 of NESA or the like which comprisesa rectangular strip which extends from the upper edge to the lower edgeof the panel and overlays the exposed cathode lines 50A, B, and C. Theanode is also in operative relation with cathode 50R.

If desired, suitable masks are provided to shield the keep-alive cell 85and reset cathode 50R from view. Such masks may be opaque films on theface plate, or they may be mechanical shields suitably disposed insidethe panel.

In panel 10, the base plate and face plate are spaced apart a distanceof the order of 20 to 25 mils, and the gas in the panel is provided at apressure of the order of 400 Torr. One suitable gas mixture comprises99.8% neon and 0.2% xenon. Another suitable gas filling is pure neon.With this arrangement of gas pressure and close spacing of base plateand face plate, as each cathode line 50 and the anode 90 are energized,cathode glow can be limited to the close vicinity of a single energizedcathode even though the cathodes are connected in groups, and, a glow istransferred from cathode to cathode, no spurious glow develops atundesired locations. This is because the ionized particles, includingmetastable states, are limited in their ability to diffuse and areneutralized by the closely spaced base plate and face plate.

One suitable circuit 92 for operating display panel 10 according to theinvention is shown in FIG. 3, wherein panel 10 is shown schematically.In the circuit, power supplies, ground connections, current-limitingresistors, and such details are omitted since such elements can bereadily provided by those skilled in the art. In addition, for morespecific circuit details, reference is made to an article entitled"Plasma's Progress: Gas Discharge Technology Moves Into Analog Realm,"which appeared in "Electronics" for March 1974.

In circuit 92, anode 90 is connected to a transistor driver 140, whichitself is connected to the output of a comparator circuit ordifferential amplifier 144 having two inputs, one from a source 150 ofan unknown analog input signal, to be represented by a bar of light inthe panel 10, and one connected to a ramp voltage generator 160.

The electrodes 50KA and 50KK of the keep-alive cell 85 are connected toa suitable source of potential V, so that this cell is always ON andgenerating excited particles. The reset cathode 50R is connected byreset bus 96R to a transistor driver 130R, and each of the groups (orphases) of display cathodes 50 is connected by a bus 96 to a transistordriver 130, with phase 1 cathodes 50A being connected by bus 96A todriver 130A, phase 2 cathodes 50B being connected by bus 96B to driver130B, and phase 3 cathodes 50C being connected by bus 96C to driver130C. A flip-flop driver 100 is provided and connected to operate eachof the drivers 130 in turn to achieve the desired scanning cycledescribed below. A clock 170 is connected to the ramp generator 160, toa flip-flop driver 100, and to a counter 180 which is adapted togenerate a number of counts or pulses equal to the number of displaycathode bars 50A, B, C in panel 10 and then to generate a reset pulsewhich is coupled to the flip-flop driver 100, to the ramp generator 160,and to the comparator 130.

In operation of panel 10, as thus connected, the keep-alive cell 85provides a constant source of excited or ionized particles, and, at thebeginning of a scanning cycle, reset cathode driver 130R is operated toapply operating potential to reset cathode 50R to cause it to glow andgenerate excited particles, and then operating potential is applied bythe other cathode drivers 130 sequentially to each of the other cathodes50 in turn, beginning with cathode 50A adjacent to the reset cathode50R. Simultaneously, operating potential is applied to the anode 90 fromthe output of the differential amplifier 144. As long as there is anoutput from the differential amplifier and the anode carries positivepotential, energization of the cathode lines 50 takes place from line toline and extends from the beginning of the series along the series untilthe analog signal and the ramp signal are equal in amplitude. At thistime, there is no output from the differential amplifier, and the anodeis de-energized; however, the clock and counter continue to run to theend of the series of cathodes to insure uniform cathode duty cycle eachtime a scanning cycle is carried out. During this time period, thecathodes glow up to a line in the cathode series determined by themagnitude of the analog signal and the length of time during which theanode is energized. Thus, a line or bar of glow is seen extending alonga length of the cathode lines representative of the amplitude of theanalog signal. This operation is repeated cyclically at such a rate thata stationary but changeable length or bar of light appears to glow inthe panel.

In the scanning sequence described above, glow is transferredpreferentially from the keep-alive cell, then to the reset cathode, thento the first display cathode 50A of the phase 1 cathodes, then to thefirst cathode 50B of the phase 2 cathodes, then to the first cathode 50Cof the phase 3 cathodes, due to the generation of excited particles,first by the keep-alive cell and then by each display cathode as it isturned on. Even though the display cathodes are connected in groups,only the cathode adjacent to an ON cathode will itself turn on, and notthe more remote cathodes of the group because they are too remote fromthe supply of excited particles. In addition, as each display cathodeturns on, the anode voltage drops (because of the current flow throughthe cell) to a level high enough to support ionization (sustainingpotential) at the ON cathode, but too low to cause ionization at anyother grounded cathode of the group.

What is claimed is:
 1. A bar graph display system comprisinga displaypanel comprising a gas-filled envelope including a base plate and aviewing face plate hermetically sealed together, a plurality of thin,parallel, linear cathodes disposed in a series along the surface of saidbase plate and positioned sufficiently close together so that, as theseries of cathodes is energized and selected cathodes are caused toexhibit cathode glow, the glow of adjacent cathodes appears to blend toform a bar of light, said series of cathodes including a first cathode,a last cathode, and intermediate cathodes, a reset cathode positionedadjacent to and ahead of the first cathode in said series for use instarting a scanning cycle for said series of cathodes, an anodeelectrode disposed on said face plate overlying said series of cathodes,conductors connected to said cathodes and connecting said cathodes ingroups with a single common conductor connected to each such group, aseparate cathode driver connected to each common conductor forenergizing each cathode in turn along said series of cathodes, a resetcathode driver connected to said reset cathode and adapted to beenergized first before said separate cathode drivers in a cathodescanning cycle, an anode driver connected to said anode electrode forapplying operating potential thereto, and a circuit coupled to saidanode driver including a signal comparator circuit having a singleoutput connected to said anode driver, said comparator circuit havingtwo inputs, one input comprising a source of a sawtooth voltage, and thesecond input comprising a source of an analog signal to be representedin said panel by a bar of light made up of a group of energizedcathodes, said cathode drivers continually energizing said cathodes insaid series and causing said cathodes to exhibit cathode glow as long asthere is an output from said comparator and said anode driver isenergized thereby and energizes said anode. .Iadd.
 2. An indicatordevice comprising an envelope containing an ionizable gas at a pressurecapable of maintaining a cathode glow discharge, at least one series ofn cathodes spaced from one another along an insulating surface withinsaid envelope, where n is an integer greater than 3, an anode in gasdischarge relationship with each cathode of the series, first circuitmeans for respectively scanning said series of cathodes, each such scanbeginning with the first cathode in the series and proceeding along thecathodes in the series one after another, second circuit meanscontrolling the voltage applied to the anode while the cathodes arebeing scanned, said second circuit means includinga counter for countingas successive cathodes are scanned and maintaining a count whichcorresponds to the cathode being scanned as the scan moves from cathodeto cathode, a ramp generator for producing a saw-tooth or ramp-shapedwaveform during each scan of the cathodes in response to a succession ofapplied input pulses, the value of the saw-tooth voltage increasing asthe counter counts, a comparison circuit having two input terminals, oneconnected to said ramp generator and the other for receiving an inputsignal having a value to be displayed on said cathode series, saidcomparison circuit applying a signal to said anode when said rampvoltage bears a predetermined relationship to said input signal, andmeans connected to the output of said counter and responsive to the fullcount thereof for applying a reset signal to portions of said secondcircuit means in advance of the initiation of a cycle of operation ofsaid first and second circuit means. .Iaddend..Iadd.
 3. An indicatordevice as in claim 2 whereinsaid counter has a total count of n and itrecycles for each new scan of the cathodes. .Iaddend. .Iadd.
 4. Anindicator device as in claim 2 wherein said cathodes are interconnectedinto n circuits, where n is 3 or more, each said circuit including adifferent group of every nth cathode, and wherein said first circuitmeans includes a drive circuit for energizing said n circuits insequence and successively repeating said sequence, further includinginput circuit means connected to said drive circuit and said counter..Iaddend..Iadd.
 5. An indicator device as in claim 2 and including ananode driver coupled between said comparison circuit and said anode..Iaddend.